Oh my. You say he wasn't in water - could he have overheated under a warm light? If a turtle falls on his back and can't flip himself over, I think they have trouble breathing and can overheat.
Are there any electrical items that could have shocked him? Like a broken water heater or light bulb, for instance.
Because turtles' metabolisms are slow, they can appear to be almost dead and then recover. Don't give up on him - just keep him warm (but not hot) and quiet and see what happens. If you think he's overheated, dip him in some tepid water to cool him off, but don't leave him where he could drown if he's having trouble moving.
Here's some info on various health conditions. Perhaps you'll find something helpful here:
http://www.redearslider.com/index_health.html
Good luck. Keep us posted as to how he does.